When a failure and outage of some power system components occur, and when all the corrective and control procedures do not maintain the system stability and equilibrium between the electrical power supply and demand, the load shedding, as the last treatment, is unavoidable. On the other hand, recent researches show that large blackouts are usually occurred after a chain of cascading outages where outage of high congested transmission lines are common components of these chains. Thus, as an optimal load shedding policy, besides of the economic aspects, optimization of the transmission reliability margin (TRM) in the emergency condition can perform a better sustainability for the power system and would prevent the probable large blackouts by terminating the next cascading outages. In this thesis, the usage contribution of various loads from the power transmission capacity in the ordinary condition and their contribution on the transmission reliability margin in emergency condition are proposed as two operational indices for priority ranking of system loads for optimal load shedding in composite power systems. The proposed methods are used for priority ranking of loads in two standard test systems of RBTS and RTS, and the obtained results are analyzed and compared from various points of view. Key words: 1-Load-shedding 2-Transmission cost allocation 3-Transmission reliability margin